You must download the Garmin Connect app to initially connect Garmin to NikePlus, but you can sync your activity using the app or Garmin Express (desktop experience).

In the Nike Run Club for iOS, open the menu tray and tap "Settings"

Tap "Partners"

Tap the Garmin icon

Tap "Connect"

If prompted, enter your Garmin login information

Tap "Allow"

Your Garmin workouts will now appear as Nike Run Club app workouts in your Nike Run Club activity history and will earn NikeFuel. They will be designated in your activity history with a small Garmin icon.